In addition to the benefits of patients with allergies and renal insufficiency, CO2 is extremely safe in a variety of arterial and venous applications. It is 20 times as soluble as room air and is expired through the lungs in a first-pass-type effect. The current contraindication to CO2 digital subtraction angiography is that the cerebral arterial circulation should never be exposed to CO2 because of possible neurotoxicity. Relative contraindications include use in the presence of a large arteriovenous shunt, with nitrous oxide anesthesia, and used cautiously in patients with chronic obstructive pulmonary disease.